1. Field of the Invention
The present invention relates to an apparatus and a method for exhaust gas purification in an internal combustion engine with a three-way catalyst disposed in an exhaust gas passage, and more particularly to an air-fuel ratio control for improving the exhaust gas purification immediately after the engine is started.
2. Related Art of the Invention
There is an earlier air-fuel ratio control technology for improving exhaust gas purification immediately after an engine is started where, by reason of a conversion rate of NOx in a catalyst being suddenly lowered more in a lean range of an air-fuel ratio than a stoichiometric value thereof immediately after the engine is started, a target air-fuel ratio in the air-fuel control is shifted more to the rich side than the stoichiometric value, thereby improving the conversion rate of NOx.
There is another earlier air-fuel ratio control technology (Japanese Unexamined Patent Publication No. 7-189768) where an air-fuel ratio is controlled to a lean range immediately after an engine is started and to a rich range at an acceleration time.
Namely, a feedback control is performed at the lean range for a predetermined period immediately after the engine is started and when an acceleration condition is detected during the feedback control in the lean range, the air-fuel ratio is controlled to be rich.
There is a further earlier air-fuel ratio control technology (Japanese Unexamined Patent Publication No. 7-301140) where an air-fuel ratio is controlled to a stoichiometric value immediately after an engine is started, then to a lean range, and thereafter, to the stoichiometric value.
Namely, a feedback correction value (before becoming lean) is stored by feedback-controlling the air-fuel ratio to the stoichiometric value immediately after an engine is started and the air-fuel ratio is feedforward-controlled to the lean range until activation of a catalyst is carried out and subsequently a feedback correction value (after becoming lean) is stored by feedback-controlling the air-fuel ratio to the stoichiometric value.
A target value for a lean air-fuel ratio by a feed forward control at the lean range is corrected by learning control with a present feedback correction value (before becoming lean) and a prior feedback correction value (after becoming lean).
There is a further earlier air-fuel control technology (Japanese Unexamined Patent Publication No. 9-242528) where an air-fuel ratio in an exhaust manifold before a catalyst changes depending on introduction of a secondary air thereto. That is, the air-fuel ratio in the exhaust manifold before the catalyst is controlled to a lean range at a cold catalyst, and then, the air-fuel ratio therein is controlled to a rich range after activation completion of the catalyst.
Namely, at the cold catalyst (from part activation to complete activation), the air-fuel ratio in the exhaust manifold is controlled to the lean range and sulfate is removed by controlling the air-fuel ratio therein to the rich range after the activation completion.
There is a further earlier air-fuel ratio technology (Japanese Unexamined Patent Publication No. 9-151759) where an air-fuel ratio is controlled to a lean range until catalyst activation is carried out and after the catalyst activation is carried out, the air-fuel ratio is controlled to a stoichiometric value.
Namely, the air-fuel ratio is controlled to the lean range when a water temperature does not reach a catalyst activation temperature and after it has reached the catalyst activation temperature, the air-fuel ratio is controlled to the stoichiometric value.